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Finding Current in a Thevenin Circuit

  1. Feb 21, 2017 #1
    1. The problem statement, all variables and given/known data

    Given a circuit with a total of 3 sources , 2 Voltage sources and 2 Current sources and 4 resostors, Find the Cirrent I2 passing through the branch where R2 and V2 are. And I am asked to give the formula for the value of the voltage in the current source. Attached the picture of the circuit .
    Now, the problem is simple I dont know if I should I take the load just as R2 or as R2 plus the voltage source V2. and I am having trouble on how to get the voltage of the current source
    2. Relevant equations
    R thevenin and V thevenin
    3. The attempt at a solution
    for the thevenin resitance I turned Isources= open loop, Vs= short circuit, ad got an Rth =R1 but Im not sure if that is right.
    for the thevenin voltage... I took just R2 as my load and when I take out the load and fond the voltage drop for U2 wich is in the same branch it gives me a non sense
     

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    Last edited: Feb 21, 2017
  2. jcsd
  3. Feb 21, 2017 #2

    gneill

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    Staff: Mentor

    Hi miguelcard,
    Welcome to Physics Forums!

    It's not clear to me how useful Thevenin's Theorem will be for this problem since it appears that you need to find circuit values (a voltage and a current) in two different locations in the circuit. In using Thevenin you have to decide on where the load is and everything else gets reduced to a single voltage and resistance; you lose access to all the individual components and circuit paths inside the "black box".

    If it were my choice I'd probably go for superposition theorem or nodal analysis.

    Were you given any component values for the circuit or are you meant to give symbolic results only?
     
  4. Feb 21, 2017 #3
    Hi thanks for your reply! ... The exercise is meant to be solved by showing the symbolic equations only, and yea I know how to find the values through nodal analysis, but this exercise says "find current I2 strictly using a thevenin equivalent" and then as a minor point it asks for "Value of UI ? (formula)" so I thought this voltage value of a current source could be found easily after, but I dont know how to do it this way
     
  5. Feb 21, 2017 #4

    gneill

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    Staff: Mentor

    Okay, well then I suppose you can treat it as essentially two separate problems.

    For the first problem (finding I2), your approach looks okay. But check your Thevenin resistance: you said it was equal to R3 in your post above, but I don't see that in your suppressed source circuit:

    upload_2017-2-21_15-3-20.png

    Can you find the potential drop across R1 when the load is removed?
     
  6. Feb 21, 2017 #5
    I mean Rth= R1 sorry about that, because the current will go through the short circuit instead of going through the other 2 resistors making it the total resistance, correct me if Im wrong, and now that you say taking them like different loads, for finding the current I2 would it be better to take the whole branch R2 + Vsource U2 as a load or just R2?
     
  7. Feb 21, 2017 #6

    gneill

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    Staff: Mentor

    I'd just take R2 as the load.
     
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